A Local Area Network (LAN) is generally defined as a broadcast domain. All nodes in a network connected to the same LAN communicate directly with each other, whereas two nodes connected to different LANs require a router to communicate. A LAN may consist of a single physical segment, or several segments interconnected by hubs, bridges or switches. A Bridged LAN (BLAN) consists of LAN segments interconnected by bridges. As data moves through a BLAN there may be multiple paths the data may take to arrive at the same destination. To avoid the problem of loops being created in a BLAN, bridges execute one of various protocols that determine which ports (points of physical access) of a bridge should be used for accepting data frames and which ports should be blocked. One such protocol is a Spanning Tree Protocol (STP) which is explained in detail in IEEE 802.1D-1998 and herein incorporated in its entirety by reference. The STP determines for each port of a bridge whether such port should be placed in a blocking mode, where no data frames are accepted or sent by the port, or placed in a forwarding mode, where data frames may be sent and received by said port. The broadcast domain for a BLAN is the entire spanning tree. In other words, whenever a frame of data is sent to an unknown address, the frame will be sent out on every LAN segment and received at all end nodes in the spanning tree.
A single BLAN can support several logical LANs, or Virtual LANs (VLANs). A VLAN consists of a subset of the BLAN. Associated with each VLAN is a VLAN ID (VID). A Virtual Bridged LAN (VBLAN), is a BLAN that allows for the definition, creation and maintenance of VLANs. A bridge that performs the necessary functions is a VLAN-aware bridge. VBLANs consist of interconnected VLAN-aware bridges. Nodes may send and receive frames associated with different VLANs; each frame that is exchanged via a VBLAN is associated to a single VLAN.
FIG. 1 depicts a typical VBLAN 100 associated with the prior art. The VBLAN 100 includes a plurality of user computer stations 110, printers 120, personal computers 130 and one or more network host computers 140 and 150. Messages, frames and data within the VBLAN 100 are passed from one network device to another via a series of bridges 160 and interconnecting links 190. End devices (such as user computer stations 110, printers 120 and the like) can be grouped into a VLAN 180 (the subset denoted by dashed line circles) for accessing minicomputer 140. Using this VLAN 180, devices are grouped logically into a single broadcast domain. Accordingly, broadcast traffic is confined to just those devices that need to see it, thereby reducing traffic to the rest of the network. All devices that exchange frames via a certain VLAN are said to be members of that VLAN.
For a BLAN, the broadcast domain consists of the entire spanning tree. The broadcast domain of a VLAN is reduced to a subset of the entire spanning tree. Such a revised broadcast domain is realized by means of limiting the VLAN membership of ports on the bridges 160. Frames belonging to a certain VLAN are only exchanged via ports that are configured to be members of that VLAN. This configuration can be static or dynamic. Dynamic configuration is achieved by using the GARP VLAN Registration Protocol (GVRP). GVRP is an application of the Generic Attribute Registration Protocol (GARP). GARP is defined in IEEE 802.1D-1998 and GVRP is defined in IEEE 802.1Q-2003 both of which are incorporated in their entirety by reference. By using GVRP, VLAN-aware bridges register and propagate VLAN memberships on all ports that are part of the active topology of the underlying spanning tree(s). For example, a frame from VLAN 180 received on a port which is a member of VLAN 180 is only forwarded on ports which are members of the same VLAN 180.
It is understood that in a sophisticated interconnection of various network devices to create VLANs, it is sometimes required to upgrade the software on a bridge to provide the best possible operating conditions within the network. Currently, upgrading of a bridge results in that bridge not sending out messages in accordance with GARP AND GARP applications (e.g., GVRP). Additionally, such bridge will not respond to such messages from neighboring bridges. Consequently, if a bridge is running the GARP application GVRP, connectivity for the VLAN is lost and the traffic from end-users is affected.